One characteristic drawback of NTSC signal format modulation schemes is that only one of the color components is subjected to vestigial sideband filtering. The I color component is accorded a bandwidth of 1.2 MHz, while the Q o color component is accorded a 0.6 MHz bandwidth. The reason for this characteristic is that it is necessary in order to separate the I and Q color components. This is accomplished by looking for the I component in the bandwidth beyond 0.6 MHz, for example. (In order to know the angle of a vector, there must be two vectors. Two distinguishable sidebands provide the two vectors and consequently the angle between them.)
Thus, quadrature modulation schemes, except those that are performing line averaging, such as the phase alternation line (PAL) scheme, are condemned to use two sidebands for the color subcarrier. And, if a vestigial sideband system is being used, as with the NTSC color signal format, vestigial sideband filtering may be performed only upon one of the two sidebands; otherwise there is no mechanism for separating the sidebands or recognizing which one is e.g. The I component and which one is e.g. The Q component.
While the disparity in bandwidth between the I and Q components is a fundamental weakness of the NTSC color signal format, a number of processes have been available to attempt to correct for this weakness. For example, chroma bandwidth expansion techniques may be employed in an attempt to restore missing bandwidth to the chroma (and sharpness to the chroma component of the picture display). While chroma bandwidth expansion is a relatively straightforward technique, there is another, and much more difficult drawback with NTSC color television signals which have been subjected to filtering at the transmission end of the path.
These filters have very sharp skirts in order to limit the effective picture energy radiated from the transmitting antenna to the allotted channel bandwidth (typically 6 MHz in the United States). These filters are consequently sometimes referred to as "brick wall" filters, and they are most effectively present at 4.2 MHz above the assigned picture carrier frequency (in order to make spectral room for the 4.5 MHz FM modulated sound carrier). Brick wall filters of the type employed in television transmission paths have the drawback of group delay errors which are most pronounced in the 3.8 MHz to 4.2 MHz range.
Group delay errors of these brick wall filters have a very severe and adverse impact upon the I and Q quadrature modulation scheme, centered at 3.579545 MHz. This impact is excessive chroma ringing in the chroma signal recovered at the receiver/display.
Thus, the compromise reached by the National Television Standards Committee (NTSC) in establishing the I lower sideband bandwidth at 1.2 MHz, and the Q upper and lower sideband bandwidth at 0.6 MHz has resulted in serious weaknesses in the NTSC signal format: after the transmission process, there is a loss of resolution (which can be overcome somewhat by chroma crispening techniques at the receiver/display), and ringing of chrominance transitions. This ringing is most difficult to reduce or eliminate at the receiver/display.
Thus, since the inception and adoption of the NTSC color signal format, there has been a need for a mechanism for distinguishing between the I and Q color component high frequencies which does not limit the Q color component to a 0.6 MHz bandwidth only.
There is also a need to reduce chrominance ringing due to brick wall filtering in the composite video path.
In the PAL color signal format, two equal color components have bandwidths of 1.5 MHz, and the components are inverted in phase from line to line. When two lines are combined without phase delay, the U color component is recovered. When the two lines are combined with a 180.degree. phase delay, the V color component is recovered. The PAL system works very well and provides greater overall color bandwidth than has heretofore been provided with the NTSC color signal format. In the SECAM color format, line sequential schemes are used, also with greater success than has heretofore been achieved with the NTSC color signal format.
While many formats and schemes may be imagined which result in improved overall chroma bandwidth and which present reduced ringing following the transmission path, a primary consideration is that the new scheme must be downwardly compatible with existing vestigial sideband filters and with existing receiver/display devices without objectionable impairment.
Thus, a hitherto unsolved need has arisen for a modified NTSC color signal format which effectively increases the overall chroma bandwidth while at the same time results in significantly reduced chroma ringing.